More profane Netspeak variation of smh, meaning "shaking my motherfucking head". An expression of general disdain for an event or statement.

IM conversation:

Userguy1: So... do you shower every day?
Usergirl25: smmfh...
Userguy1: ...
Userguy1: ...what was that for?
Usergirl25: What kind of question was that? Of *course* I shower every day! Don't you?
Userguy1: ...